Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual frequência atualmente possui 0 684?
 ###Tabelas: CREATE TABLE new_zealand_track__&__field_championships (new_zealand_track__&__field_championships_id AUTO_INCREMENT PRIMARY KEY, Year INT, Place VARCHAR, Event VARCHAR, Performance VARCHAR, Age (years) INT) CREATE TABLE film (film_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Deceased Spouse VARCHAR, Cause of Death VARCHAR, Date of Spouses Death VARCHAR, Length of Marriage VARCHAR, Children Together VARCHAR, Current Marital Status VARCHAR) CREATE TABLE defunct_callsigns (defunct_callsigns_id AUTO_INCREMENT PRIMARY KEY, Callsign VARCHAR, Area served VARCHAR, Frequency VARCHAR, Band VARCHAR, Freq currently VARCHAR, Purpose VARCHAR) CREATE TABLE club_honour_board (club_honour_board_id AUTO_INCREMENT PRIMARY KEY, Year INT, President VARCHAR, (M) Finishing position VARCHAR, (M) Coach VARCHAR, (M) Best & Fairest VARCHAR, (M) Leading Goalkicker VARCHAR) CREATE TABLE category_leadership_table (category_leadership_table_id AUTO_INCREMENT PRIMARY KEY, Stage INT, Winner VARCHAR, General classification Żółta koszulka VARCHAR, Mountains classification Klasyfikacja górska VARCHAR, Intermediate Sprints Classification Klasyfikacja najaktywniejszych VARCHAR, Points classification Klasyfikacja punktowa VARCHAR)  
SELECT Freq currently FROM defunct_callsigns WHERE Frequency = 0 684